Random access memory architecture including primary and signal bit lines and coupling means therefor

ABSTRACT

A differential latching inverter uses a pair of cross-coupled inverters having a skewed voltage transfer function to rapidly sense a differential signal on a pair of bit lines in a random access memory and provide high speed sensing during a read operation. The differential latching inverter may also include a pair of symmetrical transfer function output inverters and additional pull-up circuits to enhance high speed operation. The differential latching inverter may be used in a memory architecture having primary bit lines and signal bit lines, with a differential latching inverter being connected to each pair of signal bit lines. The primary bit lines and signal bit lines are coupled to one another during read and write operations and decoupled from one another otherwise. The read and write operations may De internally timed without the need for external clock pulses in response to a high speed address change detection system, and internal timing signals generated by delay ring segment buffers. A high speed, low power random access memory may thereby be provided.

This application is a divisional of copending application Ser. No.07/708,459, filed May 31, 1991, now U.S. Pat. No. 5,304,874.

FIELD OF THE INVENTION

This invention relates to semiconductor memory devices and moreparticularly to high speed, high density, low power random accessmemories.

BACKGROUND OF THE INVENTION

Read/write memories, also referred to as Random Access Memories (RAM)are widely used to store programs and data for microprocessors and otherelectronic devices. The availability of high speed, high density and lowpower RAM devices has played a crucial role in the price reduction ofpersonal computers and in the integration of computer technology intoconsumer electronic devices.

A typical RAM includes a large number of memory cells arranged in anarray of rows and columns. Each memory cell is typically capable ofstoring therein a binary digit, i.e. a binary ONE or a binary ZERO. Eachrow of the memory cell array is typically connected to a word line andeach column of the memory cell array is typically connected to a pair ofbit lines. Read and write operations are performed on an individual cellin the memory by addressing the appropriate row of the array using theword lines and addressing the appropriate cell in the addressed rowusing the bit lines. Depending upon the signals applied to the bitlines, a write operation may be performed for storing binary data in theRAM or a read operation may be performed for accessing binary data whichis stored in the RAM. When read and write operations are not beingperformed, the RAM is typically placed in an idle operation formaintaining the binary data stored therein.

RAMs are typically divided into two general classes, depending upon theneed to refresh the data stored in the RAM during the idle state. Inparticular, in a Dynamic Random Access Memory (DRAM), the data stored inthe memory is lost unless the memory is periodically refreshed duringthe idle operation. In contrast, in a Static Random Access Memory (SRAM)there is no need to refresh the data during an idle operation, becausethe data stored therein is maintained as long as electrical power issupplied to the SRAM. In the present state of the art, it is generallypossible to fabricate higher density DRAM arrays than SRAM arraysbecause the individual memory cells of a DRAM include fewer transistorsthan the individual cells of an SRAM. However, SRAMs tend to operate athigher speeds than DRAMs, because there is no need to refresh the datastored therein. Accordingly, both SRAMs and DRAMs are typically used incomputer systems, with the SRAMs being used for high speed memory (oftenreferred to as "cache" memory), while the DRAM is typically used forlower speed, lower cost mass memory.

Three general design criteria govern the performance of random accessmemories. They are density, speed and power dissipation. Densitydescribes the number of memory cells that can be formed on a givenintegrated circuit chip. In general, as more cells are fabricated on aVery Large Scale Integration (VLSI) chip, cost is reduced and speed isincreased.

The performance of random access memories is also limited by the powerconsumption thereof. As power consumption increases, more sophisticatedpackaging is necessary to allow the integrated circuit to dissipate thehigh power. Moreover, high power circuits require expensive powersupplies, and limit applicability to portable or battery powereddevices.

Finally, speed is also an important consideration in the operation of arandom access memory because the time it takes to reliably access datafrom the memory and write data into the memory is an important parameterin the overall system speed. It will be understood by those having skillin the art that the parameters of speed, density and power dissipationare generally interrelated, with improvements in one area generallyrequiring tradeoffs in one or more of the other areas.

In designing high density, high speed, low power random access memories,two general design areas may be pursued. The first is the design of thememory cell itself. For example, in a static random access memory,improved memory cell designs can permit high speed memory operations atlow power consumption. One such improved design is described incopending application Ser. No. 07/619,101, now U.S. Pat. No. 5,222,039,entitled Static Random Access Memory (SRAM) Including Fermi ThresholdField Effect Transistors, by the present inventor Albert W. Vinal andassigned to the assignee of the present invention. A high density, highspeed, low power SRAM cell is described.

A second major area in designing a high speed, high density, lower powerrandom access memory is the design of the supporting circuits whichallow reading of data into, writing of data from, and operationalcontrol of, the random access memory array. These circuits for reading,writing and controlling the operation of the RAM cell array are oftencritical limitations in the design of a high speed, high density, lowpower random access memory.

One particular criticality in the design of random access memory is thesense circuitry which is used to detect a binary ONE or binary ZERO fromone or more cells in the random access memory during a read operation.Known sensing designs are slow, power hungry, and have consumed adisproportionate amount of chip "real estate" (area). In particular, alinear analog sense amplifier is typically used to amplify the signalfrom a selected cell in the memory in order to detect a binary ONE orbinary ZERO, which is typically represented by a particular voltagelevel at the output of a selected cell.

In order to properly sense one of two voltage levels at the output of aparticular cell, linear analog sense amplifiers typically require areference or bias voltage, midway between the two voltage levels. Seefor example U.S. Pat. No. 4,914,634 to Akrout et al. entitled ReferenceVoltage Generator for CMOS Memories. Unfortunately, reference voltagegenerating circuits typically consume relatively large amounts of poweron the integrated circuit and also take up critical chip area.

Linear analog sense amplifiers have also required equalization of thebit lines prior to sensing, in order to prevent an imbalance in the bitlines from producing false data values. See for example U.S. Pat. No.4,893,278 to Ito entitled Semiconductor Memory Device IncludingPrecharge/Equalization Circuitry For The Complementary Data Lines.Unfortunately, the need for equalization adds to the complexity of thecircuitry on the memory. Equalization also generally requires balancedtransistors in the entire memory, thereby requiring tighter transistortolerances and lowering the yield of the integrated circuit devices.

High gain, high speed linear sense amplifiers have reduced tolerance forimbalance, thereby decreasing the number of cells that can be coupled tothe sense amplifier and further limiting the density of the memoryarray. The linear sense amplifier also limits the speed of the memorybecause linear sense amplifiers are limited by a given gain-bandwidthproduct, so that the higher the gain required, the slower the speed ofthe linear sense amplifier and vice versa.

Since linear sense amplifiers consume high power, many memory designsdeactivate the sense amplifiers when a read operation is not beingperformed. Unfortunately, deactivation reduces the speed of the memorydevice because the sense amplifiers must be reactivated prior to a readoperation.

Finally, at some point during the linear amplification of a read signal,the linearly amplified signal must be nonlinearly converted into abinary ONE or ZERO. Accordingly, the output of a sense amplifier istypically coupled to a latch, to thereby produce one or the other binarystate. See for example U.S. Pat. No. 4,843,264 to Galbraith entitledDynamic Sense Amplifier For CMOS Static RAM, and U.S. Pat. No. 4,831,287to Golab entitled Latching Sense Amplifier. Unfortunately, senseamplifiers which include a combination of a linear analog senseamplifier and a nonlinear latch are complicated and are difficult toaccurately control for high speed operation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved sense circuit for use in a memory.

It is another object of the invention to provide a sensing circuit whichis capable of sensing binary data at high speed and with low powerdissipation.

It is yet another object of the invention to provide a high speed, lowpower random access memory design.

These and other objects are provided according to the present inventionby a Differential Latching Inverter (DLI) which is responsive to thevoltage on a pair of differential inputs thereto. The DifferentialLatching Inverter (DLI) may be connected to a pair of bit lines in amemory array, for sensing the binary state of the state of a selectedmemory cell. When one of the input signals to the DLI rise above apredetermined threshold, the DLI is responsive to a small differentialcomponent between the signals applied thereto to rapidly latch theoutput of the inverter to one logical state or another. For example, ina memory using five volt and ground reference voltages, when an inputsignal to the DLI is above one volt, and an input differential of atleast two millivolts is present between the input signals, the DLIrapidly latches up to a first or a second logical value depending uponwhich of the inputs has the higher input differential.

The Differential Latching Inverter of the present invention may beimplemented using a minimal number of field effect transistors, asdescribed below, and does not require the generation of a separatereference voltage or require high gain analog linear sense amplifiersfor operation. Accordingly, high speed, low power, high density sensingof signals stored in a random access memory is provided.

A basic design of a Differential Latching Inverter of the presentinvention includes a pair of complementary field effect transistorinverters, each of which is connected between first and second referencevoltages, typically the power supply voltage V_(DD) and ground, witheach inverter including an input and an output. According to theinvention, the FETs of each of the first and second complementaryinverters are designed to produce an inverter transfer function which isskewed toward one of the first or second reference voltages. In otherwords, the inverters dc not produce a symmetrical inverter transferfunction relative to the first and second reference voltages. Rather,the transfer function is skewed toward one of the reference voltages. Ina preferred embodiment, the voltage transfer function is skewed towardsground by a factor of 21/2 less than a symmetrical inverter, so that avoltage threshold of about one volt causes the inverter to rapidlychange state, upon sensing a voltage differential of about twomillivolts.

The first and second skewed inverters of the present invention are crosscoupled by connecting the input of the first inverter to the output ofthe second inverter and the input of the second inverter to the outputof the first inverter, to thereby create a latch. A first bit line isconnected to the input of the first inverter and a second bit line isconnected to the input of the second inverter.

The Differential Latching Inverter (DLI) of the present inventionexhibits three states. When one or the other input to the DLI risesabove the threshold voltage and an input differential of two millivoltsor greater is found between the two bit line inputs, the DLI latches toa binary ONE or binary ZERO state. In a third or reset state, in whichthe bit line inputs thereto are both below the DLI's threshold voltage,both outputs of the DLI are ZERO. No DC power is dissipated by the DLIin either of its three stable states, and minimal power is dissipated bythe DLI when it switches from one state to another.

The skewed transfer function, first and second complementary invertersof the DLI may be produced by controlling the dimensions of thecomplementary FET transistors of the skewed inverters so that theproduct of the square channel saturation current and the ratio ofchannel width to length of the FETs of a first conductivity type issubstantially greater than the product of the square channel saturationcurrent and the ratio of the channel width to length of the FETs of thesecond conductivity type. Preferably, the products of the square channelsaturation current and the ratio of channel width to length differ by afactor of ten.

In a particular embodiment of the DLI, a pair of pull-up FETs may alsobe provided, with the controlled electrodes (source and drain) of afirst pull up FET being connected between the first reference voltageand the output of the first complementary FET inverter, and thecontrolled electrodes of a second pull-up FET being connected betweenthe first reference voltage and the output of the second complementaryFET inverter. The controlling electrode (gate) of the first pull-up FETis connected to the output of the second complementary FET inverter andthe controlling electrode of the second pull-up FET is connected to theoutput of the first complementary FET inverter. These cross coupledpull-up FETs increase the latching speed of the DLI.

The output of the first and second complementary inverters may becoupled to a third and a fourth complementary FET inverter,respectively. The third and fourth inverters produce an inverter voltagetransfer function which is symmetrical between the first and secondreference voltages. The outputs of the differential latching inverterare the outputs of the third and fourth complementary FET inverters.

The DLI may also include a second pull up circuit, which is connected tothe outputs of the first and second skewed transfer function inverters,for rapidly pulling the outputs of the first and second inverters to thefirst reference voltage (V_(dd)), and thereby pulling the outputs of thethird and fourth symmetrical transfer function inverters to the secondreference voltage (ground) in response to an input signal appliedthereto. The input signal is applied immediately upon a successful dataread, or immediately upon verification of a successful data write, torapidly bring the DLI to the third (reset) state and prepare the DLI fora next read or write operation. External clock timing is not required.Rather, the reset set is initiated internally, upon completion of a reador write operation.

The Differential Latching Inverter of the present invention may be usedin a high speed, high density, low power random access memoryarchitecture as follows. An array of memory cells is arranged in aplurality of rows and columns, with a word line connected to each rowand a pair of primary bit lines connected to each column. Signal bitlines are provided, orthogonal to the primary bit lines, and arespective pair of signal bit lines is connected to at least onerespective pair of the primary bit lines. A DLI is connected betweeneach pair of signal bit lines.

The primary bit lines are coupled to a first reference voltage,typically power supply voltage V_(DD), during the idle operation, and aselected one of the primary bit line pairs is decoupled from the firstreference voltage during a write operation. The signal bit lines arecoupled to a second reference voltage, preferably ground, during an idleoperation and are decoupled from the second voltage during a read orwrite operation. The primary bit lines and the signal bit lines arecoupled together during read and write operations and decoupled from oneanother during an idle operation.

Accordingly, during an idle operation each of the primary bit line pairsis referenced to V_(DD) and each of the signal bit line pairs isreferenced to ground. All of the DLIs are in their third or reset state.In order to read, the signal bit lines are decoupled from the secondvoltage reference source (ground) and the primary bit lines remaincoupled to the first voltage reference source (V_(DD)). A word decoderselects a given row. A bit decoder couples a primary bit line pair in aselected column to its associated signal bit line pair. The amount ofvoltage delivered to one bit line or the other of the selected primarybit line pair drops more rapidly than the other due to the currentconducted by one of the memory cell pass transistors, as controlled bythe state of the selected memory cell being read. This currentdifferential translates to a voltage differential on one or the other ofthe signal bit lines of the associated signal bit line pair. When thevoltage differential on one of the signal bit lines exceeds the DLI'sthreshold voltage, the DLI will rapidly latch into one or the otherstate depending on the signal bit line which had the higher voltage.Accordingly, high speed sensing of data read from a random access memoryis provided with minimal supporting circuitry.

The outputs of all of the DLIs may be directly connected to a pair of ORgates, with the output of one OR gate signifying that a logical ONE hasbeen read and the output of the second OR gate signifying that a logicalZERO has been read. Connection of all of the DLIs to a single OR gatefor reading is possible because all of the DLIs which are not being readare in their third or reset state with both outputs thereof at groundpotential. The output of the activated DLI may be placed in a readregister and provided as the memory output. Once a DLI has been latchedand the data has been read, the memory is rapidly restored to the idlestate by pulling the active DLI back to its idle state. The signal bitlines are recoupled to ground, the primary bit lines remain coupled toV_(DD) and the signal bit lines and primary bit lines are decoupled fromone another. Accordingly, a self-timing operation is provided.

In a write operation, a word decoder selects a given row, a selectedpair of primary bit lines is decoupled from V_(DD) by a decoded writegate, and one selected primary bit line pair is coupled to anappropriate signal bit line pair. One of the signal bit lines is clampedat a LOW level thereby forcing the associated primary bit line towardsground. This forces one side of the selected memory cell towards groundwhile holding the other side to greater than V_(DD) /2, thereby storingdata into the selected RAM cell. At the same time, the data written intothe selected memory cell is also read by the associated DLI as describedabove. The successful read causes the memory to be reset in its idlestate as described above.

According to another aspect of the present invention a circuit may beused with the DLI and memory architecture described above, to detect anaddress change at the memory input and initiate a read or writeoperation. The address change detection system uses a transitiondetection delay unit for each address bit of the memory. The transitiondelay unit is responsive to a change in its associated address bit toprovide a clock output pulse of predetermined duration.

The transition detection delay unit comprises a latch which is coupledto the associated address bit, and a pair of Delay Ring Segment Bufferseach coupled to a respective output of the latch. The design andoperation of the Delay Ring Segment Buffer is described in copendingapplication Ser. No. 07/497,103, now U.S. Pat. No. 5,030,853, entitledHigh Speed Logic and Memory Family Using Ring Segment Buffer by thepresent inventor Albert W. Vinal, assigned to the assignee of thepresent invention, the disclosure of which is hereby incorporated hereinby reference. The output of the delay ring segment buffer is provided tocascaded NAND gates to form the output of the transition detection delayunit.

The outputs of all of the transition detection delay units are providedto an OR gate which is preferably a Complementary Logic Input Parallel(CLIP) OR gate, as described in application Ser. No. 07/648,219, nowU.S. Pat. No. 5,247,212, entitled Complementary Logic Input Parallel(CLIP) Logic Circuit Family by the present inventor Albert W. Vinal andassigned to the assignee of the present invention, the disclosure ofwhich is incorporated herein by reference. The output of the CLIP ORgate provides an indication of an address change. Accordingly, thetransition detection delay unit uses simple circuitry to detect anaddress change, with less time delay than known address change detectioncircuits. Similar transition detection is employed to detect a chipselect active transition and a write enable transition. The outputs ofthese transition detect delay units are also coupled to the CLIP ORgate, and are also used to activate the memory cycle.

Once a change in the address has been detected, or a chip select orwrite enable signal has been detected, internal timing of the memory maybe provided by a series of Delay Ring Segment Buffers. The Delay RingSegment Buffers provide the required timing signals to word and bitdecoders and the DLIs as described above. Once the data has been read,or data has been written and verified, the timing circuitry generates areset signal to rapidly place the memory in the idle state. Self-timingof memory operations is thereby provided.

It will be understood by those having skill in the art that theDifferential Latching Inverter of the present invention may be used inconjunction with other memory architectures than described herein.Similarly, the memory architecture described herein may be used withsensing circuits other than the Differential Latching Inverter. Finally,the unique control circuits such as the address detection changecircuits and the timing circuits using ring segment buffers, may be usedto control memories other than those described herein. However, it willbe also be understood by those having skill in the art that the uniquecombination of the DLI, memory architecture and supporting controlcircuitry described herein provides a high density, high speed randomaccess memory with very low power dissipation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic circuit diagram of a DifferentialLatching Inverter according to the present invention.

FIG. 2 illustrates the inverter transfer functions of the symmetricalinverters and the skewed inverters of the Differential Latching Inverterof FIG. 1.

FIGS. 3A-3D illustrate timing diagrams for operation of the DifferentialLatching Inverter of FIG. 1.

FIGS. 4A and 4B, which form FIG. 4 when placed adjacent one another asindicated, illustrate a block diagram of a random access memoryarchitecture according to the invention incorporating the DifferentialLatching Inverter of FIG. 1.

FIG. 5 illustrates a schematic circuit diagram of read and write controlcircuits for a random access memory according to the present invention.

FIG. 6 illustrates a schematic circuit diagram of a data input registerfor a random access memory according to the present invention.

FIG. 7 illustrates a schematic circuit diagram of timing controlcircuitry for a random access memory according to the present invention.

FIG. 8 illustrates a block diagram of an address change detectioncircuit according to the present invention.

FIG. 9 illustrates a block diagram of an alternative address changedetection circuit according to the present invention.

FIG. 10 illustrates a timing diagram for operation of the address changedetection circuits of FIGS. 8 and 9.

FIGS. 11A and 11B are truth tables to illustrate the operation of theaddress change detection circuits of FIGS. 8 and 9 respectively.

FIGS. 12A and 12B are a circuit schematic diagram of the address changedetection circuitry of FIG. 9.

FIG. 13 is a timing diagram for a random access memory according to thepresent invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which a preferred embodimentof the invention is shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiment set forth herein; rather, this embodiment is provided so thatthis disclosure will be thorough and complete, and will fully convey thescope of the invention to those skilled in the art. Like numbers referto like elements throughout.

The design and operation of the random access memory of the presentinvention will be described by first describing the DifferentialLatching Inverter (DLI). The overall architecture of the memory arrayincluding the Differential Latching Inverter will then be described,followed by the operation of the memory during idle, read and writecycles. The control circuits for performing the read, write and idleoperations will then be described.

Differential Latching Inverter

Referring now to FIG. 1, a Differential Latching Inverter (DLI)according to the present invention will now be described. As shown inFIG. 1, DLI 10 includes a pair of cross coupled, skewed transferfunction complementary field effect transistor inverters 11, 11'. Themanner in which the skewed transfer function inverters are designed willbe described below. When the input signals on one of bit lines 20 or 20'rise above the DLI's threshold voltage, and a small differential signalcomponent, for example at least two millivolts, is present, a binaryoutput latchup condition rapidly occurs that produces a binary ONE valueat one of output terminals 27, 27' of the DLI and a binary ZERO value atthe other one of output terminals 27, 27' of the DLI. The binary signalstate of the selected RAM cell being read is determined by which outputterminal 27, 27' of the DLI is HIGH.

The skewed inverters 11, 11' are connected between a first referencevoltage 14 (here shown as power supply voltage V_(DD)) and a secondreference voltage 15 (here shown as ground). The input 12, 12' of arespective inverter 11, 11' is connected to a respective one of a pairof bit lines 20, 20'. As also shown in FIG. 1, the skewed complementaryinverters 11, 11' are cross coupled, with the output 13 of inverter 11being connected to an input of inverter 11' and the output 13' ofinverter 11' being connected to an input of inverter 11.

It will be understood by those having skill in the art that skewedcomplementary inverters 11, 11' may be formed using a pair ofcomplementary (i.e. N-channel and P-channel) field effect transistors,with the inverter input being the gates of the transistors and thesources and drains of the transistors being serially connected betweenpower supply and ground, and the inverter output being the connectionnode between the field effect transistors. However, a preferredembodiment of the skewed inverters 11, 11' is as illustrated in FIG. 1.As shown, each inverter comprises a first conductivity (P-channel)transistor 21, 21' and a pair of second conductivity (N-channel)transistors 22, 22' and 23, 23' respectively The controlled electrodesof these transistors (drains and sources) are serially connected betweenthe power supply 14 and ground 15. The gates of transistors 21 and 22are coupled to bit line 20 and the output of the inverter 13 is theconnection node between P-channel transistor 21 and N-channel transistor22. Similar connections apply to inverter 11'. In order to cross couplethe inverters, the output 13 of inverter 11 is coupled to the gate oftransistor 23' and the output 13' of inverter 11' is coupled to the gateof transistor 23.

DLI 10 also includes an optional pair of symmetrical transfer functioninverters 16, 16' with each symmetrical inverter 16, 16' comprising apair of complementary transistors 24, 24' and 25, 25', connected betweenpower supply voltage 14 and ground 15. The input 17, 17' of thesymmetrical inverter 16, 16, is connected to the respective output 13,13' of the skewed inverter 11, 11'. The outputs 18, 18' of thesymmetrical inverter 16, 16' form the outputs 27; 27' of the DLI. Themanner in which symmetrical inverters 16, 16' are designed will bedescribed below.

DLI 10 also includes optional pull-up circuit 19. As shown, pull-upcircuit transistors 26, 26' are connected between power supply 14 andthe respective output 13, 13' of skewed inverter 11, 11'. The gates ofpull up transistors 26, 26' are cross-coupled to the respective output13, 13' of the skewed inverter 11, 11'.

Still referring to FIG. 1, an optional second set of 29, 29' of pull-uptransistors is provided. Each optional second pull-up circuit 29, 29,includes a pair of transistors 30, 30' and 31, 31', serially coupledbetween power supply voltage 14 and a respective output 13, 13' of theskewed inverter 11, 11'. As shown, the gate of one transistor 30, 30' isconnected to the respective bit line 20, 20' and the gates of the othertransistors 31, 31' are coupled together to form a memory operation(MOP) input 28. The operation of this MOP input will be described indetail below. Briefly, during read or write operation, the MOP input 28is high so that it doesn't effect operation of the DLI. However, at theconclusion of a read or write operation, the MOP input 28 is brought LOWto turn on the pull-up circuit 29, 29', and rapidly force nodes 13, 13'to V_(DD), thereby forcing DLI outputs 27, 27' to ground.

Referring now to FIG. 2, the inverter transfer functions of symmetricalinverters 16, 16' and skewed inverters 11, 11' are shown. As shown, theoutput voltages (at nodes 13, 13,) of the skewed inverters 11, 11' areskewed towards the second reference potential 15 (i.e. ground) relativeto the input voltages thereof (at nodes 12, 12'). In particular, forreference voltages of 5 volts and ground, the output voltages of skewedinverters 11, 11' rapidly change state at an input voltage of about onevolt. Stated differently, the output voltage is skewed by a factor of21/2 less than a symmetrical inverter. This contrasts with the invertertransfer function of the symmetrical inverters 16, 16', the outputvoltages of which (at nodes 18, 18') change state symmetrically about aninput voltage (at nodes 17, 17') approximately midway between the firstreference voltage 14 and the second reference voltage 15. For five voltand ground reference voltages, the symmetrical inverters switch state atabout 2.5 volts.

Left hand skewing of inverters 11, 11' accomplishes two primary results.First, it allows DLI 10 to sense a voltage differential on bit lines 20,20' immediately after one of the bit lines rises above the noise level.Sensing not need to wait until the bit lines rise to half the powersupply voltage. Second, it causes the slope (voltage gain) of thetransfer function at the skewed switching point to be much higher thanit is at the midway point. Compare the slopes of the two curves of FIG.2. Rapid latchup is thereby provided.

Left hand skewing of the voltage transfer function of inverters 11, 11'is accomplished by making the product of the N-channel transistor (22,22', 23, 23') maximum square channel saturation current (I*satN) and thechannel width-to-length ratio of the N-channel transistors substantiallylarger than the product of the P-channel square channel saturationcurrent (I*satP) and the channel width-to-length ratio of the P-channeltransistors 21-21'. It will be understood by those having skill in theart that the square channel saturation current is the maximum currentwhich can be produced by a channel having equal length and width. Thesquare channel saturation current is proportional to the value of thecarrier mobility in the respective transistor; i.e. the electronmobility in the N-channel transistor and the hole mobility in theP-channel transistor. Since the channel lengths of all FET transistorsin a typical integrated circuit are generally made equal, above therelationship may be generally represented as:

    (I*satN)(z.sub.N)>>(I*satP)(Z.sub.P)

Preferably the product of saturation current and channel width of theN-channel devices is made ten times greater than that of the P-channeldevices. For silicon devices having equal channel lengths, the relativechannel widths of the P-channel devices 21' and the N-channel devices22, 22', 23, 23' are shown in FIG. 1 inside the respective transistors.These channel widths can be scaled to any desired groundrules.

As also shown in FIG. 2, inverter 16, 16' has a symmetrical voltagetransfer function. This is obtained by making the product of the squarechannel saturation current and the width-to-length ratio of theP-channel transistors substantially equal to that of the N-channeltransistors. Since for silicon, the P-channel transistor has a squarechannel saturation current about half that of a N-channel transistor,the symmetrical transfer function is obtained by making the channel theP-channel transistor twice as wide as the N-channel transistor. Therelative dimensions are shown in each transistor in FIG. 1.

Differential Latching Inverter Operation

Operation of the Differential Latching Inverter (DLI) 10 of FIG. 1 willnow be described. In general, when the input signal on one of bit lines20, 20' rises above the DLI's threshold voltage, the DLI outputs 27, 27'rapidly latch to represent one or the other binary signal state.Specifically, when one of the signals on the bit lines 20, 20' is abovethe threshold voltage of the DLI, and a small differential signalcomponent, for example of at least two millivolts, is present, a binaryoutput latchup condition rapidly occurs that produces a binary ONEsignal at one output terminal 27, 27' of the DLI and a binary ZERO(down) signal at the other output 27, 27' of the DLI. The binary signalstate of the selected memory cell being read is determined by whichoutput terminal 27, 27' of the DLI is HIGH. For example, when output 27goes up to V_(DD) a binary ONE has been read from memory, and whenoutput 27' goes up to V_(DD) a binary ZERO has been read from memory.

The DLI has a third or reset state that occurs when both outputs 27 and27' are at DOWN level (i.e. at or near ground level). The third state isautomatically set when the bit lines 20, 20' are both at or near groundpotential. When the DLI is not being called to read or write, both ofthe bit lines 20, 20' are placed at ground potential so that both outputterminals 27, 27' are at LOW output state, i.e. at ground. It will beunderstood by those having skill in the art that substantially no DCpower is dissipated by DLI 10 in any of the three stable states. Minimalpower is dissipated only during the switching interval; i.e. whenswitching from one state to another. The amount of power dissipated is afunction of the switching frequency.

During a read operation, a selected bit line pair is coupled to a singlememory cell selected by a word line. Once coupled together, the voltageon bit lines 20, 20' both ramp-up from ground. However, the ramp-up rateis faster on one bit line than the other bit line as a function ofwhether the selected memory cell is storing a binary ONE or ZERO.

It will be recalled that the inverter transfer function of inverters 11,11' is skewed towards ground potential. For example, voltage leveltransfer may occur at around one volt. Accordingly, assume that thevoltages on bit lines 20 and 20' are increasing from ground, but thatthe voltage on bit line 20 is increasing from ground at a slightlyfaster rate due to the binary value stored in the selected RAM cell.When the voltage on bit line 20 exceeds one volt, the output 13 ofinverter 11 rapidly switches LOW (to ground potential), forcing theoutput 13' to remain HIGH (near V_(DD)). Since output 13 is at groundpotential, the input to cross-coupled transistor 23' is also at groundpotential turning off transistor 23' and thereby forcing node 13' toV_(DD). Accordingly, latch-up rapidly occurs.

In summary, the DLI includes a feedback mode of operation which resultsin a high gain rapid latching condition determined by the imbalance ininput (bit line) ramp-up voltage rates. A two millivolt differencebetween the input signals above threshold is sufficient to cause thedesired latch-up state. The sensitivity of the DLI to the RAM cell stateto induce a differential signal component during a read cycle isprimarily due to the heavily left hand skewed voltage transfer functionin the inverters 11, 11'.

The first pull-up circuit 19 increases the latch-up speed of DLI 10. Inparticular, if bit line 20 first exceeds threshold and the output 13 ofskewed inverter 11 is first forced to ground, transistor 26' of pull-upcircuit 19 is turned on, thereby also rapidly bringing (or holding) node13' to V_(DD). Since node 13' is HIGH, transistor 26 is turned off anddoes not pull node 13 up. Accordingly, pull-up circuit 19 increases thespeed at which latch-up occurs.

It will be assumed for the present that MOP input 28 is at HIGH logiclevel so that transistors 30, 31', 31 and 31' are off and the secondpull-up circuits 29, 29' are not operational. Second pull-up circuits29, 29' are used to restore the third or reset state of the DLI at theconclusion of a read or write operation, as will be described in detailbelow.

It will also be understood by those having skill in the art thatsymmetrical inverter 16, 16' may be used to provide an output 27, 27'for the DLI which is a TRUE output (as opposed to a COMPLEMENT output)of the sensed signal. In other words, if the voltage in bit line 20increases faster than 20', the latchup will force output 27 HIGH and 27'LOW. It will also be understood that inverters 16, 16' should have asymmetrical voltage transfer function so that they latch up rapidly whenoutput nodes 13, 13' of the skewed inverters change state.

Referring now to FIGS. 3A-3D, the above described operation isillustrated. Voltage wave forms for the bit lines 20 and 20' and theoutputs 27, 27' of the skewed inverters 11, 11' are shown. As shown inthe first time interval for FIGS. 3A-3D, when the input on bit lines 20,20' are below about one volt, the outputs 27, 27' remain at ground.However, as shown in the first time interval of FIG. 3A, when thevoltage on bit line 20' is greater than about one volt and exceeds thevoltage on bit line 20 by about two millivolts, line 27' rapidly latchesto 5 volts and the slight rise in line 27 is immediately suppressed bythe feedback condition. During a data read operation latchup occurs inabout 1.65 nanoseconds from the start of the word pulse, using 0.8micron groundrules. The second time interval of FIGS. 3A-3D illustratesthe latchup of output 27 in response to the voltage on bit line 20 beinghigher than that of bit line 20'. After sensing of the stored dataoccurs, the voltage on both outputs are rapidly brought to ground byoperation of the MOP input 28 which will be described below.

Memory Architecture Incorporating The DLI

Having described the design and operation of the DLI, a high speed, lowpower, high density memory architecture which uses the DLI will now bedescribed. This architecture will be described relative to an SRAM,however it will be understood by those having skill in the art that thearchitecture may also be used in a DRAM.

Referring now to FIGS. 4A and 4B, which are placed together as indicatedto form FIG. 4, random access memory (RAM) 40 comprises an array of RAMcells 41. It will be understood by those having skill in the art thatRAM cells 41 may be SRAM cells or DRAM cells, and may use cell designswell known to those having skill in the art. As illustrated in FIG. 4,RAM cells 41 are configured in an array of m rows and n columns. Forexample, in a 128 k bit RAM, 256 rows and 512 columns of RAM cells maybe used. As also shown, m word lines 42a-42m are coupled to a one-of-mrow decoder 43 for accessing one of word lines 42a . . . 42m. As alsoshown in FIG. 4, n pairs of bit lines 44a, 44a'-44n, 44n' are connectedto the respective n rows of the array. As will be described below, twosets of bit lines are used in RAM 40, so that bit lines 44 are referredto as the "primary" bit lines.

Still referring to FIG. 4, it may be seen that p pairs of "signal" bitlines 45a, 45a'-45p' are provided, with every p'th pair of primary bitlines 44 being connected to a respective one of the signal bit lines 45.In the example shown herein, p=16, i.e. 16 pairs of signal bit lines 45,45' are provided, with every 16th column being connected to a respectiveone of the bit lines. In other words, bit line pairs 44₁, 44₁ ', 44₁₇,44₁₇ '. . . 44₅₉₇, 44₅₉₇ ' are connected to signal bit lines 45a, 45a ',and bit lines 44₁₆, 44₁₆ '. . . 44₃₂, 44₃₂ '. . . 44₅₁₂, 44₅₁₂ ' areconnected to signal bit line pair 45p, 45p'. The signal bit lines aregenerally orthogonal to the primary bit lines.

The choice of the number of signal bit line pairs depends on severalfactors. In particular, it has been found that the total capacitancewhich loads the primary bit lines 44 should be equal to or greater thanthe total capacitance loading the signal bit lines 45. The totalcapacitance which loads the signal bit lines 45 is primarily due to thediffusion capacitance of the coupling transistors which couple theprimary and signal bit lines, as described below. It has been found thatthis loading capacitance should be minimized to achieve the maximummemory clock rate and minimum data access time and is inverselyproportional to the number of DLI 10 used to configure the system.Finally, the relationship between m (the number of rows), n (the numberof columns), and p (the number of DLIs) will also depend on the overallconfiguration of the RAM

Continuing with the description of FIG. 4, a DLI 10a . . . 10p isconnected to a respective signal bit line 45a . . . 45p. First, secondand third coupling means, 46, 47 and 48 respectively, are used toselectively couple the primary bit lines 44 to the first referencepotential 14 (V_(DD)), to selectively couple the signal bit lines 45 tothe second reference potential 28 (ground), and to selectively couplethe primary bit lines 44 to the signal bit lines particular, the firstcoupling means comprises n pairs of P-channel transistors 49a, 49a'-49n,49n, for coupling a respective primary bit line 44a, 44a . . . 44n, 44n'to V_(DD) under control of gate inputs 51a-51n. Second coupling means 47comprises p pairs of N-channel FETs 51a, 51a'-52p, 52p', each of whichcouples a respective signal bit line 45a, 45a'-45p, 45 p' to ground 28under control of gate 53. Finally, third coupling means 48 is seen toinclude P-channel transistors 54a, 54a-54n, 54n' for coupling a primarybit line 44a, 44a-44n, 44n' to a respective signal bit line 45a,45a'-45p, 45p' under control of gate 55a-55n. An N-channel transistor56a, 56a-56n, 56n' also couples a respective primary bit line 44a, 44a44a'-44n, 44n' to a respective signal bit line 45a, 45a-45p, 45p' undercontrol of gates 571-57n.

As will be seen from the operational description below, the firstcoupling means 46 couples the primary bit lines to V_(DD) during theidle operation and during the read operation and decouples at least oneof the primary bit line pairs from V_(DD) during a write operation. Thesecond coupling means 47 couples the signal bit lines to ground duringthe idle operation and decouples the signal bit lines from ground duringa read operation and a write operation. The third coupling means 48couples the primary bit lines to the signal bit lines during a read andwrite operation and decouples the primary bit lines and signal bit linesfrom one another during an idle operation. In particular, P-channeltransistors 54 couple the primary bit lines to the signal bit linesduring read operation and N-channel transistors 56 couple the primarybit lines to the signal bit lines during a write operation.

Operation of the Random Access Memory

The detailed operation of the random access memory 40 (FIG. 4) will nowbe described. The idle state will first be described followed by theread state and then the write state.

During the idle state, a LOW logic level is provided to gates 51 offirst coupling means 46 to turn all of transistors 49 on and therebyplace the primary bit lines 44 at the power supply level V_(DD). At thesame time, a HIGH logic level is provided to input 53 to turn on secondcoupling means 47, and thereby couple all of the signal bit lines 45 toground. A high logic level is applied to inputs 55 and a low logic levelis applied to inputs 57 to thereby turn transistors 54 and 56 off andthereby decouple the primary bit lines 44 from the signal bit lines 45.Finally, since all of the signal bit lines 45 are at ground, all of theDLIs 10 are in their third or idle state with all of the outputs 27 and27' being at ground potential. No DC power is consumed by the circuitduring the idle state.

During a read operation, row decoder 43 selects one of word lines 42a .. . 42m to access a particular row of RAM cells 41. A logic LOW signalis applied to input 53 to turn second coupling means 47 off to therebydecouple signal bit lines 45 from ground. Although not coupled toground, the capacitance of the signal bit lines maintains the signal bitlines near ground potential. A logic LOW level is maintained at gates 51to thereby continue to couple the primary bit lines to V_(DD). A columndecoder, not shown in FIG. 4, provides a LOW logic level to a selectedone of inputs of 55a-55n depending upon the column to be read. Thisturns on the appropriate transistor pair 54, 54' and causes current toflow between the associated primary bit lines 44, 44', and the signalbit lines 45, 45'.

It should be noted that FETs 54 are connected as current controlleddevices, the current through which is controlled by their sourcevoltage. Accordingly, the primary bit line which is at a higher voltagewill produce more current to pull up the signal bit lines, than theprimary bit line which is at a lower voltage. Since the selected RAMcell current tries to discharge one or the other side of the primary bitlines 44, 44', the voltage of one of the primary bit lines drops fromV_(DD) at a rate faster than the other, depending on the state of theselected RAM cell 41. Current flows between the selected primary bitline pair 44, 44', and the signal bits lines causing a difference tooccur in the voltage ramp-up rate on the signal bit line pair 45, 45'.When the ramp-up voltage on one or the other of the signal bit lines 45,45, exceeds the threshold of the DLI 20, the output of the DLI israpidly latched to a ONE or ZERO. In other words, either output 27 goesHIGH and 27' goes LOW or output 27' goes HIGH and 27 goes LOW.

As described in detail below, the outputs 27 of all of the DLIs may begated (ORed) together because all of the DLIs which are not active arein their third state. Accordingly, the output of the activated DLI maybe placed in a read register and provided as the chip output, asdescribed in detail below.

Once a DLI has been latched and the data has been read, the RAM israpidly restored to the idle state by activating the MOP input 28(FIG. 1) with a logic LOW signal, to immediately pull the DLI back toits idle state. At the same time, once the data has been read, a HIGHsignal is applied to input 53 to thereby reactivate second couplingmeans to return the signal bit lines to ground and a HIGH signal isapplied to input 55 to decouple primary bit lines 44, 44' from signalbit lines 45, 45'. Once this has occurred, the MOP input 28 is againbrought HIGH to disable the second pull-up circuit 29 because the DLI isnow in the reset state. The operation of the control circuits forrestoring the RAM after a read operation will be described in detailbelow.

From the above description it may be seen that the read operation isself-timing. In other words, once the data has been read, the RAM resetsitself to the idle state without the need for a reset clock pulse.Accordingly, speed is not hampered by clocking requirements, andoperations can occur as fast as possible consistent with reliablereading of data. The DLI also provides reliable reading of data at highspeed, so that high speed operation of RAM 40 may be obtained.

In the write operation, a selected one of inputs 51a-51n is placed HIGHby a column decoder to thereby deactivate the associated first couplingmeans 46 and thereby decouple the associated pair of primary bit lines44, 44' from V_(DD). A HIGH logic signal is applied to select one ofinputs 57a-57n to thereby couple the selected primary bit lines 44, 44'to the appropriate signal bit lines 45, 45'. One of the signal bit linesis clamped at LOW level which thereby forces one of the selected primarybit lines to ground. This forces one side of the selected RAM cell toground and causes the other side to go up thereby storing data in theselected cell. During the write operation, transistors 54 are maintainedoff and transistors 52 are turned off to decouple the signal bit linesfrom ground. After the write operation is successfully performed, thewritten data is automatically sensed by the associated DLI, and thememory is reset as described above for the read operation. The operationof the control circuits for restoring the RAM after a write operationwill be described in detail below.

Having described the general operation of the RAM of the presentinvention, the detailed circuitry for controlling the operation of theRAM will now be described.

Read and Write Control Circuit

Referring now to FIG. 5, there is illustrated a schematic circuitdiagram of the circuit for coupling each of p signal bit line pairs 45a,45a-45p, 45p' to a DLI 10a-10p and coupling the outputs 27, 27' of eachDLI to a data output register. Circuitry for referencing the signal bitline pairs 41a, 41a'-45p, 45p' to ground is also shown along withcircuitry to control the binary value written into a selected RAM cell41 from a given signal bit line pair.

Referring again to FIG. 5, each of the output terminals 27, 27' of a DLI10, for example, output terminals 27p, 27p' of DLI 10p, is shown coupledto a p-input Complementary Logic Input Parallel Clocked OR gate 61, 61'also referred to as a CLIP-C OR gate. The CLIP-C OR gate is described indetail in copending application Ser. No. 07/648,219, now U.S. Pat. No.5,247,212, entitled Complementary Logic Input Parallel (CLIP) LogicCircuit Family by the present inventor Albert W. Vinal and assigned tothe assignee of the present invention, the disclosure of which isincorporated herein by reference. Conventional cascaded OR gates mayalso be used; however, as described in the aforesaid copendingapplication, a single CLIP-C OR gate can handle large numbers of inputsat high speed and low power.

As shown, outputs 27₁ -27_(p-1) and 27₁ '-27_(p-1) ' of the remainingDLI circuits 10₁ -10_(p-1) drive other input terminals of these CLIP-COR gates. The logic output 78, 78' of each CLIP-C OR gate drives theinput of a transfer memory (TRAM) output cell 61 comprising a pair ofcross-coupled complementary inverters, via coupling transistors 63, 63'.As shown, if output 27p of DLI 10p is HIGH, then N-channel transistor 63is turned on and the left side of TRAM cell 61 is driven LOW.Alternatively, if output 27p' of DLI 10_(p) is HIGH, then N-channeltransistor 63' is turned on via CLIP OR gate 61' and the output of TRAMcell 62 is HIGH. The clock inputs 75, 75' to CLIP-C OR gates 61, 61'will be described below, in connection with FIG. 7. The outputs 78, 78'of OR gates 61, 61' are also provided to reset circuit 88 of FIG. 7, vialines 77, 77' as described below.

As shown, the output 64 of TRAM cell 62 is coupled to a ring segmentbuffer 65 having four stages, to allow the output of the TRAM cell torapidly drive off-chip or on-chip load capacitance with a specifiedvoltage rise and delay time. The ring segment buffer design is describedin application Ser. No. 07/497,103 entitled High Speed Logic and MemoryFamily Using Ring Segment Buffer by the present inventor Albert W. Vinalassigned to the assignee of the present invention and now U.S. Pat. No.5,030,853, the disclosure of which is hereby incorporated herein byreference. The output 66 of the ring segment buffer 65 is the digitaldata output of the memory array.

Accordingly, during a read operation, one output of one DLI will goHIGH, as a function of the voltage ramp differential on the associatedsignal bit line. One input to OR gate 61, or one input to OR gate 62will thereby go HIGH. One of OR gate outputs 78 or 78' will thereby goHIGH, thereby setting or resetting TRAM 62. The output of TRAM 62 drivesring segment buffer 65, to thereby provide a HIGH or LOW data input. Thering segment buffer 65 may be configured as a tristate driver, undercontrol of a chip select signal, in order to accommodate a plurality ofRAM outputs on a single bus.

Still referring to FIG. 5, when the RAM is in its idle state, the gatesof transistors 52p, 52p' are HIGH because the MOP gate 28 is LOW causingthe output 53 of complementary inverter 69 to be HIGH. The gate inputterminals of the transistors in inverter 69 are driven by the MOP gate28. Generation of the MOP signal is described in detail below. In theabsence of a MOP gate 28, each bit line of all signal bit line pairs iscontinually referenced to ground by transistors 52, 52'. Voltagereferencing is terminated only when a MOP gate is active.

During a write interval, transistors 67, 67' and 71 provide means forcontrolling the binary state written into a selected RAM cell. A RAMcell selection occurs at the intersection of a selected word line 42 anda selected primary bit line pair 45 (FIG. 4). The gate input terminalsof transistors 67, 67', are coupled through a logic AND gate (notshown), to the ONE and ZERO output terminals respectively, of a binarydata input register described below in connection with FIG. 6.

During a write interval, the gate input 68 to transistor 71 is broughtHIGH, thereby clamping the common source connection between transistors67 and at ground potential. Transistor 71 allows one or the other bitline of a signal bit line pair to be clamped to ground, depending onwhether the gate voltage is applied to transistor 67 or 67'. If the datainput register contains a binary ONE, then transistors 67 and 71conduct, clamping the ZERO side 20 of the signal bit line pair toground. At the same time, the ONE side of the signal bit line pair 20'is not clamped to ground. The opposite conditions exist if the datainput register produces an UP level voltage at the gate of transistor67, and a DOWN voltage at the gate of transistor 67.

FIG. 6 illustrates the data input register 70. As shown, a data input 76to the RAM array is coupled to a transfer memory output cell 73, theZERO output of which is coupled to a first ring segment buffer 74 andthe ONE output of which is coupled to a second ring segment buffer 74'to produce a ZERO output 72' or a ONE output 72 which is coupled to theinput 72, 72' of FIG. 5. The ring segment buffer is described in theaforesaid application Ser. No. 07/497,103, now U.S. Pat. No. 5,030,853.It allows a given load to be driven, with a predetermined rise time, andminimum delay.

The data input register circuit 70 allows a slow rise time input to beconverted into fast rise time TRUE and COMPLEMENT outputs, with aminimum delay. Accordingly, the circuit of FIG. 6 may also be used tobuffer slow rise time RAM inputs (such as address or select inputs), foruse in the RAM array.

Continuing with the description of the write operation, and referringagain to FIG. 4, assume that a particular primary bit line pair 44, 44'is decoded and activated by bit line decoder. Transistors 49, 49' ofthis bit line pair are turned off during a write cycle by selecting theappropriate input 51 via the bit line decoder. Appropriate decodedcoupling transistors 56, 56' are turned on. One side or the other of asignal bit line pair 45, 45' is clamped to ground by the data inputregister via transistors 67, 67' (FIG. 5). This causes the associatedtransistor 56, 56' (FIG. 4) to pull down one primary bit line 44, 44'towards ground potential. The unclamped signal bit line rapidly rises involtage until the sum of this voltage and the drop in the primary bitline voltage equals the power supply voltage V_(DD). Preferably, the RAMcell design allows the increase in the unclamped signal bit line voltageto be equal to the decrease in the primary signal bit line voltage.

During a write cycle, one of m word lines 42 is also turned on by rowdecoder 43 (FIG. 4), applying gate voltage to the pass transistors ofthe RAM cell. The selected RAM cell pass transistors thereby couple thepotential of the primary bit lines to or from a common signal point inthe RAM cell. During write, the primary bit line that is driven to nearground potential sets the state of the selected RAM cell. When the stateof the selected RAM cell is set, the MOP gate generator described belowis terminated along with the write gate 68 (FIG. 5), and transistors 49,49' are turned on to recharge the primary bit lines 44 back to powersupply voltage V_(DD). Simultaneously, transistor 71 of FIG. 5 is turnedoff and transistors 52, 52' are turned on allowing both signal bit lines45, 45' to be returned to ground potential.

During the write interval, the rising potential of the unclamped signalbit line rapidly causes the associated DLI to respond to this signalvoltage when it exceeds the threshold voltage of the DLI. The binarystate written into the RAM cell is therefore also transmitted to theoutput TRAM 62 (FIG. 5) and presented to the output 66, as describedabove for the read operation, allowing error detection functions to beperformed. It will be understood by those having skill in the art thatthe simultaneous sensing of the signal voltage written into the selectedRAM cell during a write operation allows the RAM to terminate the writeoperation without the need for external clocking. Resetting of the RAMafter a write or read operation will be described below.

Memory Operation (MOP) Timing Control

Referring now to FIG. 7, the circuitry for controlling the timing of aread and write operation, collectively referred to as a memory operation(MOP) is shown. This circuitry generates a MOP signal which is used atvarious portions of the RAM architecture as previously described.Activation of the MOP signal initiates a read or write operation, anddeactivation of the MOP signal terminates the read or write operation,as described below. By generating an internal MOP signal, and using theMOP signal to control the timing of read and write operations, thememory operation is independent of an external clock. System power isdissipated only during the MOP interval, and is primarily related to theswitching power; i.e. it is proportional to capacitance times voltagesquared times the switching frequency. When the MOP gate is off, theonly power dissipated by the system is due to transistor leakagecurrent. None of the circuits within the system dissipate standby powerwhen the memory is not functioning in a read or write mode, regardlessof whether the chip select is active or not. A low power, high speedmemory is thereby provided.

Moreover, since the memory creates its own timing signals for read andwrite operations, all timing and logic functions within the memory areautomatically temperature compensated, allowing the RAM to reliablyoperate over a broad range of temperatures. At high temperatures, themaximum access rate is lowered from room temperature due to the reducedcurrent capabilities of the transistors. At low temperatures, themaximum access rate is increased above the room temperature value due tothe increased current capabilities of the transistor.

Referring again to FIG. 7, the read/write operation timing circuitry 80is controlled by a TRAM cell 82 comprising a pair of cross-coupledinverters and a pair of pass transistors of well known design. This TRAMcell is turned on and the output 83 thereof goes HIGH when an addresschange detection system issues an address change detection clock pulseon input 85, upon detecting a change in the input address. This TRAMcell is also turned on when a chip select transition going active, or awrite enable transition going active, is detected by a TDLU discussedbelow in connection with FIG. 8. The address change detection system isdescribed in connection with FIG. 8 below.

The output 83 of RAM cell 82 is coupled to a ring segment buffer 86, theoutput of which is coupled to a group of ring segment buffers 84. Thesering segment buffers provide the mechanism for driving the total loadcapacity associated with the clock lines and the system logic cells suchas the bit and word address decoding drivers and the DLI sensingsystems. These ring segment buffers also provide the proper delay fortiming the various internal circuits in the RAM, as described below.

As shown in FIG. 7, five delay ring segment buffers 84a-84e are used,however other numbers of ring segment buffers may be used in othermemory architectures. Ring segment buffers 84a and 84b are used to clockthe bit decoders (not shown) for the primary bit line pairs, and ringsegment buffers 84c and 84d are used to clock the row decoder 43 (FigureThe input stage of each of ring segment buffers 84a-84d comprise a twoinput CMOS NAND gate. One of the input gate electrodes of this NAND gateis driven by the appropriate output of the high order bit of the m bitword and n bit address registers. The other input is driven by the MOPgate. This NAND gate permits segmenting the total number of row andcolumn selects of the RAM into at least two halves. The first halfcontains m/2 low order addresses and n/2 high order addresses.Accordingly, clocking in high order groups is inhibited when addressinglow order group selection and vice versa. This procedure eliminatesdissipating unnecessary switching power during a read or write memorycycle and simplifies the design of the clock driver. However, it will beunderstood by those having skill in the art that the word and bit decodefunctions need not be divided into groups.

The output of delay ring segment buffer is provided to the DLI input 28(FIGS. 1 and 5) and to the clock inputs of the CLIP-C OR circuits 75,75' (FIG. 5). Accordingly, after a predetermined period from the time anaddress change is detected, the DLI input 28 is activated and a clockpulse is applied to the CLIP-C OR gate. Application of the MOP input theDLI 10 of FIG. 1, allows the DLI to rapidly latch into one or the otherbinary state, without interference from the second pull-up circuit 29,Application of the MOP input to clocking inputs of the CLIP-C OR gates75 provides a clock pulse for timing the output of the CLIP-C OR gate.

Still referring to FIG. 7, two input CMOS OR gate 88 is driven by theoutputs 77, 77' of the p-input CLIP-C OR gates 61, 61' (FIG. 5). Thereset output 81 of this OR gate resets TRAM 82 and thereby resets eachring segment buffer 84 after the predetermined delay of each ringsegment buffer. After a RAM cell has been read (either during a readcycle or at the end of a write cycle) one or the other p-input CLIP-C ORgates 61, 61' (FIG. 6) will deliver a logic HIGH voltage at output 71 or71', to signal completion of the intended operation. In other words, aDLI has properly stored a bit value which was read or has properlystored a bit value which was written to confirm that writing has takenplace. When this event occurs, the MOP gate is no longer required and isautomatically terminated by action of the MOP gate reset driver 88. Allclock drivers subsequently shut down within the propagation delay timeof the ring segment buffers 84.

In particular, ring segment buffers 84a and 84b shut down the bitdecoders and ring segment buffers 84c and 84d shut down the worddecoders 43 (FIG. 4). Ring segment buffer 84c terminates the MOP signalwhich shuts off CLIP-C OR gates 61, 61' (FIG. 5) and also causes secondpull-up circuits 29, 29' (FIG. 1) to rapidly bring DLI 10 to its resetstate (both inputs at ground). A memory operation (read or write) isthereby automatically terminated.

From the above description it may be seen that the feedback shutdowncontrol of the MOP gate generator automatically accommodates broadthermal environments that the RAM may experience, since MOP shutdownoccurs only after a read or write function completion has been detectedby the DLI. In other words, the MOP gate is initiated when either anaddress change, chip select or write enable is detected, indicating thata read or write operation is to begin, and is automatically terminatedonce the proper read or write function has been completed. When neithera write or read function is required, the MOP gate is off and remainsoff until turned on again by the output of the change detector. Theaddress change detector operation will be described in the next sectionin connection with FIG. 8.

Address Change Detention System

In general, a random access memory can begin a memory operation (i.e. aread or a write operation) by detecting a change in at least one of theinput address bits. In a conventional address change detection system,the time required to detect a change in the input address cansignificantly slow the memory cycle time. According to the invention, animproved address change detection system detects a change in an inputaddress in minimum time. The system uses a transition detection logicunit (TDLU) which is shown in FIG. 8. Prior to describing the TDLU, aconventional address change detection system will be described.

There are three basic elements required in a conventional address changedetection system. The first is a latch which is used to increase therise time of the input address bit. Using the example of a memory with mrows and n columns, a total of m+n latches are required to compare them+n latches allow comparison of the m+n address bits. The secondcomponent of a conventional address change detection system is anexclusive OR circuit for each of the latches. The exclusive OR circuitwill provide an output whenever the previous address bit and the presentaddress bit are different. Finally, all of the exclusive OR gate outputsare ORed together, to provide a HIGH logic level when any of theexclusive OR gates are HIGH. A change in the address is therebydetected.

The above described exclusive OR and OR logic is responsible for most ofthe delay in detecting the change in the input address, due to the largenumber of inputs which have to be ORed together. For example, for a 64 kbit RAM, the total number of address bits (m+n) is 16, and for a 256 kbit RAM the total number of address bits (m+n) is equal to 18. Usingconventional CMOS gates, a cascaded tree of CMOS gates is required toprovide the function of a 16 or 18 input OR gate.

For example, using conventional three input CMOS OR gates, a nine-ORgate tree is necessary to OR 18 inputs. Six OR gates accept the total of18 inputs at a first level of the tree. The outputs of each group ofthree gates are provided to an OR gate at a second level. Two OR gatesare used in the second level to accept all six outputs from the firstlevel. Finally, at a third level, one OR gate combines the output of thetwo second level OR gates. Propagation delay time through this logictree is excessive and requires many transistors to perform the function.

Referring now to FIG. 8, a block diagram of the address change detectionsystem 90 of the present invention will now be described. As shown, theaddress change detection system comprises m+n Transition Detection DelayUnits (TDLU) 92a-92n. A respective address bit 91a-91n is provided asthe input to a respective transition detection delay unit 92a-92n. Therespective outputs 93a-93n of the transition detection delay units92a-92n are provided as inputs to a single m+n input Complementary LogicInput Parallel (CLIP) OR gate 102. The output 85 of CLIP OR gate 102provides an address change detection signal which is provided to the MOPgenerating circuit 80 of FIG. 7. The design and operation of acomplementary logic input parallel OR circuit 102 is described in theaforementioned application Ser. No. 07/648,219, now U.S. Pat. No.5,247,212.

Each TDLU 92 delivers a clock pulse to the appropriate input of the CLIPOR gate 101 when an address transition is detected on its input addressline 91. One TDLU is coupled to the chip select latch and one TDLU iscoupled to the write enable latch (not shown). Their outputs are alsoinputted to CLIP OR gate 102. The basic components of the TDLU are alatch 94a-94n, whose logical state is controlled by a single inputsignal line 91a-91n which is connected to the address inputs of the RAMchip. The ONE and ZERO outputs of the latch, 95a-95n and 95a-95n'respectively, rapidly switch when a transition in the input signal 91occurs and provides both the TRUE and COMPLEMENT function of the inputsignal. Identical ring segment buffers 96a-96n and 96a'-96n' are coupledto the true and complement outputs 95a-95n and 95a'-95n' of the latches94a-94n. As shown in FIG. 8, ring segment buffers 96 are delay ringsegment buffers with an odd number of stages to provide an invertingdelay ring segment buffer (RSB-I). The design and operation of a delayring segment buffer is described in application Ser. No. 07/497,103, nowU.S. Pat. No. 5,030,853. As described in this application, the delayproperty of the ring segment buffer is controlled by proper choice ofchannel length for the P- and N-channel transistors used to form thering segment buffer inverters. The outputs of the ring segment buffersand the outputs of the latch are each connected to cascaded NAND gates98a-98n as illustrated in FIG. 8, to form the output 93a-93n of theTDLUs 92a-92n.

FIG. 9 illustrates an alternative design for the TDLU 92. In thisalternative design, noninverting delay ring segment buffers, consistingof an even number of inverter stages, are used. The latch outputs 95,95' are cross-coupled with the ring segment buffer outputs in order toprovide the proper inputs to the cascaded NAND gates 98. FIG. 10illustrates the relationship between the input address bit 91 and theoutput 93 of each of the TDLUs 92, 92' of FIGS. 8 or 9. As shown, apositive going or negative going transition in an address bit 91provides a clock pulse of a predetermined duration at the output 93. Theduration of the clock pulse resulting from detecting a transition at theoutputs of the latch, is controlled by the time delay designed into thering segment buffers 96.

FIGS. 11A and 11B illustrate the truth tables for the TDLU 92 of FIG. 8and the TDLU 92' of FIG. 9, respectively. Referring to FIGS. 11A and11B, it may be seen that both configurations of the TDLU produce thesame output function for the same input function.

The address change detection system of the present invention, is simpleto construct and virtually eliminates propagation delay time required todetect a change in an input voltage function, and has broad functionalapplication for high speed computer design philosophy. It will also benoted that the TDLU technology automatically accommodates the demands ofthe MOP gate generator for temperature effects.

FIGS. 12A and 12B, which together form FIG. 12 as indicated, illustratea circuit schematic diagram of the address change detection circuitry ofFIG. 8. As shown, TRAM 92 includes latch 94 and a pair of three stage(inverting) ring segment buffers 96, 96'. Complementary Logic InputParallel NAND gates 99, 100 and 101 are also shown. Assuming equalchannel lengths, the relative channel widths of the respectivetransistors are shown within the respective transistors.

The output 93 from the transition detection delay unit 92 is provided asan input to multiple input CLIP OR gate 102. The corresponding outputsfrom the other transition detection delay units are also provided asinputs to the CLIP OR gate 102. Also provided as an input to the CLIP ORgate is a chip select input 103 so that the output 85 of CLIP OR gate102 is at logic HIGH whenever an address change is detected and the RAMchip has been selected.

Timing of RAM Operation

Having now described the individual components and the detailedoperation of the present invention, an overview of the memory timingwill now be described in connection with the timing diagram of FIG. 13.The time line of FIG. 13 is calibrated in nanoseconds and the values arebased on simulations of the RAM of the present invention, with the FETsbeing fabricated using 0.8 micron groundrules.

The timing diagram begins at time equals zero, with a change on inputaddress 91 of FIG. 8. The change in input address is detected and theoutput 85 of the address change detection system of FIG. 8 is producedafter 1.1 nanoseconds. This output is provided to the timing circuit 80of FIG. 7, and the output of ring segment buffer 84e produces the MOPsignal after about 1.75 nanoseconds. At about 3.5 nanoseconds, the bitdecoders and word decoders are clocked via the outputs of ring segmentbuffers 84a-84d of FIG. 7. Accordingly, the read or write intervalbegins after about 3.5 nanoseconds from the time the input addresschanged.

An output is produced on the DLI at just over five nanoseconds and theMOP reset signal 81 of FIG. 7 is produced shortly thereafter. The dataout signal 66 in FIG. 5 is produced approximately 2.7 nanoseconds fromthe time the read/write interval began. The reset signal propagatesthrough the ring segment buffers 84a-84e between five and sixnanoseconds to turn off the CLIP-C OR gate 75, 75' of FIG. 5 and toactivate the second pull-up circuit of the DLI via MOP input 18.Accordingly, after about seven nanoseconds, a new read/write cycle maystart with a new change in the input address.

The random access memory of the present invention may also be operatedin a unique write mode called "burst write". Burst write is achievedwhen the write enable is active, the chip select (103, Figure 2) isactive, and the transition detection delay unit output starts the memorycycle with each detected address change and the DLI output terminatesthe MOP gate. This burst write cycle can be used efficiently to fullyload all or a part of the total memory in minimal time and with minimalpower consumption.

From the above Description of a Preferred Embodiment, it will beunderstood by those having skill in the art that the DifferentialLatching Inverter, memory architecture, read and write control circuit,memory operation timing control circuit and address change detectioncircuit may be used independently to improve the operation ofconventional random access memories. However, it will also be understoodby those having skill in the art that these elements may all beincorporated together into a unique random access memory design whichexhibits high speed and low power dissipation. For example, a computersimulation of a 128 kilobit SRAM array using these circuits andimplemented in 0.8 micron MOSFET technology exhibits a read or writecycle time of eight nanoseconds, and a power dissipation of 200milliwatts operating at 125 mHz, at room temperature. The memorydissipates 200 microwatts when idle. This performance is unheard of inthe present state of the art of SRAM design. When 0.8 micron Fermi-FETtechnology is employed, 200 mHz performance is readily achieved withless power.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

That which is claimed is:
 1. A Random Access Memory (RAM) for storingtherein binary data during a write operation, for maintaining storedbinary data therein during an idle operation, and for accessing binarydata stored therein during a read operation, said RAM comprising:anarray of memory cells arranged in a plurality of rows and a plurality ofcolumns, each of said memory cells adapted for storing therein a binarydigit; a plurality of word lines, each of which is connected to arespective row of said memory cell array; a plurality of pairs ofprimary bit lines, each pair being connected to a respective column ofsaid memory cell array; a plurality of pairs of signal bit lines, eachpair being connected to at least one respective pair of said primary bitlines; a plurality of sensing means, each being connected to arespective pair of signal bit lines, for reading the binary digit storedin a selected one of said memory cells; first means for coupling saidprimary bit lines to a first reference voltage during an idle operationand during a read operation, and for decoupling a selected pair of saidprimary bit lines from said first reference voltage during a writeoperation; second means for coupling said signal bit lines to a secondreference voltage during an idle operation, and for decoupling a pair ofsaid signal bit lines which is connected to said selected pair ofprimary bit liens from said second reference voltage during a readoperation, and for decoupling only a first signal bit line of said pairof said signal bit lines which is connected to said selected pair ofprimary bit lines from said second reference voltage during a writeoperation; and third means for coupling said selected pair of saidprimary bit liens to said pair of said signal bit lines which areconnected to said selected pair of primary bit lines during a readoperation, to thereby cause current to flow from said selected pair ofsaid primary bit lines to said pair of said signal bit liens which areconnected to said selected pair of primary bit lines while said primarybit lines are coupled to said first reference voltage, and for couplingsaid pair of signal bit lines to said selected pair of primary bit linesduring a write operation, to thereby clamp a first one of said selectedpair of primary bit lines which is coupled to a second signal bit linesof said pair of said signal bit lines to said second reference voltage,and for decoupling said primary bit lines and said signal bit lines fromone another during an idle operation.
 2. The RAM of claim 1 wherein saidfirst coupling means comprises a plurality of pairs of field effecttransistors, a respective one of which is connected between said firstreference voltage and said one end of a respective one of said primarybit liens.
 3. The RAM of claim 2 further comprising means for activatingthe controlling electrodes of said plurality of pairs of field effecttransistors, to couple said primary bit lines to said first referencevoltage during an idle operation and during a read operation, and fordeactivating the controlling electrodes of at least one pair of saidplurality of pairs of field effect transistors, to decouple theassociated pair of primary bit liens from said first reference voltageduring a write operation.
 4. The RAM of claim 1 wherein said secondcoupling means comprises a plurality of pairs of field effecttransistors, a respective one of which is connected between said secondreference voltage and a respective one of said signal bit lines.
 5. TheRAM of claim 4 further comprising means for activating the controllingelectrodes of said plurality of pairs of field effect transistors tocouple said signal bit lines to said second reference voltage during anidle operation, and for deactivating the controlling electrodes of saidplurality of pairs of said field effect transistors to decouple saidsignal bit lines from said second reference voltage during a readoperation and a write operation.
 6. The RAM of claim 1 wherein saidthird coupling means comprises a plurality of pairs of field effecttransistors of first conductivity type, a respective one of which isconnected between a respective primary bit line and a respective signalbit line, and a plurality of pairs of field effect transistors of secondconductivity type, a respective one of which is connected between arespective primary bit line and a respective signal bit line.
 7. The RAMof claim 1 wherein each of said plurality of sensing means comprises aField Effect transistor (FET) Differential Latching Inverter (DLI)circuit comprising:first and second complementary FET inverters, each ofwhich is connected between said first and second reference voltages, andeach of which includes a first input, a second input and an output; theFETs of each of said first and second complementary inverters producingan inverter transfer function which is skewed toward said secondreference voltage; the second input of said first inverter beingconnected to the output of said second inverter, and the second input ofsaid second inverter being connected to the output of said firstinverter; a respective first one of each of said signal bit line pairsbeing connected to the first input of a respective one of said firstinverters, and a respective second one of each of said signal bit linepairs being connected to the first input of a respective one of saidsecond inverters; and the outputs of said first and second complementaryFET inverters producing output signals for said DLI circuit.
 8. The RAMof claim 7 wherein said first and second complementary FET inverterseach comprise at least one FET of first conductivity type and at leastone FET of second conductivity type;the product of the square channelsaturation current and the ratio of width to length of said at least oneFET of said first conductivity type being substantially greater than theproduct of the square channel saturation current and the ratio of widthto length of said at least one FET of said second conductivity type, tothereby produce said skewed inverter transfer function.
 9. The RAM ofclaim 7:wherein said first complementary FET inverter comprises a firstinverter FET of first conductivity type, and second and third inverterFETs of second conductivity type, said first, second and third FETsbeing serially connected between said first and second referencevoltages, the controlling electrodes of said first and second FETs beingconnected together to form the first input of said first inverter, thecontrolling electrode of said third FET forming the second input of saidfirst inverter; wherein said second complementary FET inverter comprisesa fourth inverter FET of first conductivity type, and fifth and sixthinverter FETs of second conductivity type, said fourth, fifth and sixthFETs being serially connected between said first and second referencevoltages, the controlling electrodes of said fourth and fifth FETs beingconnected together to form the first input of said second inverter, thecontrolling electrode of said sixth FET forming the second input of saidsecond inverter; the controlling electrode of said third FET beingconnected to a node which connects said fourth FET to one of said fifthand sixth FETs, the controlling electrode of said sixth FET beingconnected to a node which connects said first FET to one of said secondand third FETs; the FETs of each of said first and second complementaryinverters producing an inverter transfer function which is skewedtowards use of said first and second reference voltages; the secondinput of said the second input of said first inverter being connected tothe output of said second inverter, and the second input of said secondinverter being connected to the output of said first inverter; arespective first one of each of said signal bit line pairs beingconnected to the first input of a respective one of said firstinverters, and a respective second one of each of said signal bit linepairs being connected to the first input of a respective one of saidsecond inverters; and the product of the square channel saturationcurrent and the ratio of channel width to length of the second, third,fifth and sixth inverter FETs being greater than the product of thesquare channel saturation current and the ratio of channel width tolength of said first and fourth inverter FETs, to product said skewedinverter transfer function; the outputs of said first and secondcomplementary FET inverters producing output signals for said DLIcircuit.
 10. The RAM of claim 7 further comprising first and secondpull-up circuits, each of which includes an input and an output;theoutput of said first pull-up circuit being connected to the output ofsaid first complementary FET inverter, and the output of said secondpull-up circuit being connected to the output of said secondcomplementary inverter; the inputs of said first and second pull-upcircuits being responsive to an input signal to force the outputs ofsaid first and second inverters towards said first reference voltage.11. The RAM of claim 7 further comprising:third and fourth complementaryFET inverters, each of which is connected between said first and secondreference voltages and each of which includes an input and an output;the output of said first complementary FET inverter being connected tothe input of said third complementary FET inverter and the output ofsaid second complementary FET inverter being connected to the input ofsaid fourth complementary FET inverter, the outputs of said third andfourth complementary FET inverters producing inverted output signals forsaid DLI circuit; the FETs of each of said third and fourth invertersproducing an inverter voltage transfer function which is symmetricalbetween said first and second reference voltages.
 12. The RAM of claim 7further comprising:third and fourth complementary FET inverters, each ofwhich is connected between said first and second reference voltages andeach of which includes an input and an output; the output of said firstcomplementary FET inverter being connected to the input of said thirdcomplementary FET inverter and the output of said second complementaryFET inverter being connected to the input of said fourth complementaryFET inverter; the FETs of each of said third and fourth invertersproducing an inverter voltage transfer function which is symmetricalbetween said first and second reference voltages.
 13. The RAM of claim12 wherein said third and fourth complementary FET inverters eachcomprise at least one FET of first conductivity type and at least oneFET of second conductivity type;the product of the square channelsaturation current and the ratio of width to length of said at least oneFET of said first conductivity type being substantially equal to theproduct of the square channel saturation current and the ratio of widthto length of said at least one FET of said second conductivity type, tothereby produce said symmetrical inverter transfer function.
 14. The RAMof claim 1 wherein each of said third plurality of sensing meanscomprises:a plurality of Field Effect Transistors (FETs), said FETsbeing interconnected between said first and second reference voltagesand being free of connection to reference voltages other than said firstand second reference voltages; said FETs being responsive to arelatively small voltage differential between first and second signalbit lines of an associated signal bit line pair, and further beingresponsive to one of said bit lines being at a voltage which is greaterthan a predetermined threshold voltage, with said threshold voltagebeing substantially closer to said second reference voltage than to saidfirst reference voltage, for rapidly producing one of a first and asecond digital value depending upon the direction of said voltagedifferential between said first and said second signal bit lines. 15.The RAM of claim 7 further comprising first and second p-input OR gatenetworks, a respective input of said first OR gate network beingconnected to the output of a respective one of said first complementaryinverters, and a respective input of said second OR gate network beingconnected to the output of a respective one of said second complementaryinverters, said first OR gate network producing a signal at the outputthereof upon sensing of a first binary value by any of said DLIs, andsaid second OR gate network producing a signal at the output thereofupon sensing of a second binary value by any of said DLIs.
 16. The RAMof claim 15 further comprising a latch, having a pair of inputs and anoutput, a respective input of which is connected to a respective outputof said first and second p-input OR gate networks, for latching one ofsaid first and second binary values therein.
 17. The RAM of claim 16further comprising a Ring Segment Buffer, having an input and an output,the input of which is connected to said latch, and the output of whichproduces a read data output for said RAM.
 18. The RAM of claim 15wherein each of said OR gate networks comprises a single ComplementaryLogic Input Parallel (CLIP) OR gate.